The invention relates to an anti-skid brake control system for a motor vehicle having forward and rearward sets of vehicle wheels, at least one of which is driven. The anti-skid brake control system includes speed sensors associated with the vehicle wheels; an evaluation circuit responsive to the wheel speed sensors for producing brake pressure control signals; and brake pressure control means for controlling the brake pressure applied to the vehicle wheels in response to the control signals.
At a low friction coefficient, and this applies especially to a rear wheel driven vehicle, there is a situation in which, during the control of a pressure level corresponding to the locking limit of the front wheels, the momentary balance at the rear axle between braking torque and driving torque--especially at a high idle speed during cold operation--results in a positive driving torque. A consequence thereof is that the vehicle does not come to complete stop despite a locking of the front wheels. A stopping of the vehicle is only possible when the brake pressure is increased to a value which exceeds the motor driving torque. This requires in some cases significant brake pressures and hence high pedal forces.
This situation can be intensified with the combination of an anti-skid brake control system based on the feedback principle and a vacuum brake power booster. When the energy consumed by the pressure modulation is not sufficiently replaced and when the throttle is partially open, the level control point of the booster continuously decreases. In this case the foot pressure required for reaching a sufficient pressure level to surpass the driving torque increases to values which cannot be reached in borderline cases.
The above described problems can result in a situation in which a vehicle--especially with rear drive--cannot be stopped on a slippery road even when great pedal forces are applied unless the driver disengages the clutch or sets the shift lever of an automatic transmission to "neutral". However, especially in case of an automatic transmission, such a reaction cannot be expected by a driver in a "stress situation".